Replaced an xUnit.net assertion with Unquote.
Replaced an xUnit.net assertion with Unquote.

For this refactoring, I considere piping into `(=!) true`, but by using
the full `test <@ ... @>` expression, Unquote will be much better able to
progressively report on what went wrong if the test fails.

ErikSchierboom/FsCheck,ploeh/FsCheck,fscheck/FsCheck,fsharp/FsCheck,fscheck/FsCheck,jhamm/FsCheck,sideeffffect/FsCheck